EP 0 155 096 discloses the preparation of 17β-substituted 4-aza-5-alpha-androstanes having a 1,2-double bond by oxidizing the corresponding 1,2-dihydro compound by means of benzeneselenic anhydride. Further processes for introducing a 1,2-double bond in 17β-substituted 4-aza-5-alpha-androstanes are, for example, also described in EP 0 298 652, EP 0 428 366 and EP 0 473 225. 17β-substituted 4-aza-5-alpha-androstanes having a 1,2-double bond are widely used pharmaceutically active compounds. Of significance is, for example, the compound 17β-(N-tert-butylcarbamoyl)-4-azaandrost-1-en-3-one (finasteride) which is used, for example, as a 5-alpha-reductase inhibitor for the treatment of benign prostate hyperplasia or of alopecia androgenitica. Also of significance is, for example, 17β-{N-[2,5-bis(trifluoromethyl)phenyl]}-4-azaandrost-1-en-3-one (dutasteride). The known processes for preparing these compounds have specific disadvantages, so that there is a need for improved alternative processes.